Wheel-driven machines, such as dump trucks for example, are typically provided with at least one inter-axle differential, commonly referred to as an "open differential", having a carrier or differential housing and two output shafts or half shafts that each drives one wheel of the machine. As well known, the differential utilizes a gear system that permits the output shafts, and thereby the driven wheels, to rotate at different speeds, thereby reducing transmission loads and understeering during normal cornering. In a machine with front and rear driven axles, each driven axle is typically provided with an open differential, and torque is transmitted to the driven axles through a transfer case interconnecting the axles. The transfer case is generally provided with a so-called center differential to accommodate differences in rotational speed between different axles and the wheels driven thereby.
Under slippery or otherwise poor surface conditions, however, open differentials may result in a loss of traction. As well known, if one wheel or axle driven through the differential loses traction, the other wheel or axle loses input torque. Therefore, known differentials are commonly provided with a differential lock to selectively neutralize the normal differential action by locking the output shafts to the differential housing. For example, a dog-clutch locking arrangement has been provided in known spur-type differentials and includes a fixed drive dog connected to and rotating with the differential housing and a slidable drive dog rotating with but slidable along one output shaft. When the drive dogs are not mutually engaged, the differential operates as a conventional open differential. When the drive dogs are mutually engaged, the output shaft connected to the slidable drive dog is locked to the differential housing and the other output shaft is indirectly locked to the differential housing via the planetary gearing of the differential.
To control the locking and unlocking of the differential, the slidable drive dog is connected to the plunger of a hydraulic actuator mounted on the differential casing. The drive dogs are disengaged by applying hydraulic pressure ahead of a piston fixed to the plunger to retract the plunger and separate the drive dogs. To lock the differential, the hydraulic pressure is removed, and a spring forces the piston and the plunger to an extended position whereby the slidable drive dog is moved into engagement with the fixed drive dog, thereby locking the differential.
A problem arising in the use of such known locking arrangements is that, when the hydraulic pressure ahead of the piston is removed, viscous drag on the piston from the hydraulic fluid and the high effective inertia of the fluid in the hydraulic system results in relatively slow movement of the piston and plunger and, therefore, the slidable drive dog. This is especially the case when the hydraulic fluid ahead of the piston is cold. Accordingly, the slidable drive dog is slow to fully engage the fixed drive dog or the drive dogs will not fully engage at all.